1. Field of the Invention
The present invention relates to a magnetoresistive sensor, a thin-film magnetic head, a head gimbal assembly, and a hard disk device, and more particularly, to a magnetoresistive sensor for use in thin-film magnetic heads of magnetic storage apparatuses such as a hard disk device and the like.
2. Description of the Related Art
To accommodate the trend of increasingly higher a real density for magnetic recording, magnetic heads using GMR (Giant Magnetoresistive) sensors have been developed for use as a read element. Among others, a GMR sensor using a spin valve (SV) film can provide a magnetic head having a higher sensitivity by virtue of its large change in magnetoresistance to a sense current applied to the device for reading data recorded on a recording medium. Here, the SV film refers to stacked layers which comprise a ferromagnetic layer that has the direction of magnetization fixed in one direction (hereinafter also referred to as a “pinned layer”), a ferromagnetic layer that changes the direction of magnetization in accordance with an external magnetic field generated by a recording medium (hereinafter also referred to as a “free layer), and a non-magnetic intermediate layer interposed between the two ferromagnetic layers. In a SV film, the magnetization direction of the free layer makes an angle relative to the magnetization direction of the pinned layer, in accordance with the external magnetic field, such that spin dependent scattering of conductive electrons vary in accordance with the relative angle to cause a change in magnetoresistance. A magnetic head detects this change in magnetoresistance to read magnetic information on the recording medium.
While prevalent in MR device using a SV film is a CIP (Current in Plane)-GMR sensor in which a sense current flows in parallel to the layers, in order to accommodate a further increase in areal density, development has been recently advanced for a magnetic head using a CPP (Current Perpendicular to the Plane)-GMR sensor in which a sense current flows perpendicularly to the layers. While CPP type sensor includes a TMR (Tunnel Magneto-resistance) sensor using a TMR layer, a CPP-GMR sensor is expected as a sensor having a high potential because of its lower resistance as compared with a TMR sensor, and its ability to generate higher output power for data read even from a narrow track as compared with a CIP-GMR sensor.
However, if an SV film having a stack configuration similar to that of a CIP-GMR sensor is simply applied to a CPP-GMR sensor, the resulting CPP-GMR sensor cannot provide a sufficient change in magnetoresistance. This is mainly because the resistance of portions (free layer, pinned layer, and non-magnetic intermediate layer) contributing to a change in magnetoresistance occupies only a small proportion in the overall resistance of the device. Specifically, a CIP-GMR sensor can ensure a sufficient change in magnetoresistance in the in-plane direction due to spin dependent scattering on the layer boundaries, since the sense current conducts in the in-plane direction of the layers. Whereas in a CPP-GMR sensor, since the sense current flows perpendicularly through the layers, i.e., layer boundaries, it causes only insufficient spin dependent scattering on the boundaries. In addition, since conventional GMR sensors have only two boundaries, one is between the non-magnetic intermediate layer and the free layer and the other is between the non-magnetic intermediate layer and the pinned layer, the boundaries contribute less to the change in magnetoresistance. These are considered as a major factor. On the other hand, in a CPP-GMR sensor, since the sense current flows through each layer, scattering of conducting electrons within the each layer, i.e., bulk scattering is generally larger than a CIP sensor, thus lending itself to contribution to a larger change in magnetoresistance. For this reason, in a CPP-GMR sensor, thicker free layer and pinned layer are effective for ensuring a larger change in magnetoresistance.
Alternatively, instead of increasing the thicknesses of the free layer and pinned layer of a SV film, a non-magnetic intermediate layer may be inserted within the free layer or the pinned layer to increase the number of boundaries, thereby enhancing the magnetoresistive effect, as disclosed, for example, in the specification etc. of Japanese Patent Laid-open Publication No. 2003-152239. This specification etc. discloses a free layer configuration which is comprised of a stack of a nickel iron ally CoFeB, non-magnetic layer Cu, and a cobalt iron alloy (CoFeB/Cu/CoFeB stack configuration). Such a layer configuration can provide a larger change in magnetoresistance, because there is larger spin polarization on the boundaries between the CoFe-based alloy layers and Cu layer to promote the spin dependent scattering.
Further, in a free layer of a CIP-GMR sensor, there is disclosed a layer configuration comprised of a stack using CoFe/NiFe and a Cu layer, (NiFe/CoFe/Cu/CoFe/NiFe layer configuration). See, for example, in the specification etc. of Japanese Patent Laid-open Publication No. 2003-8103. The stack of CoFe and NiFe can provide a larger spin polarization on the CoFe/Cu boundaries, together with soft magnetic characteristics of NiFe.
In this way, while the free layer and the pinned layer tend to be thicker in a CPP-GMR sensor, a reduction in noise and consistent stability are important requirements in the free layer, thus rendering the soft magnetic characteristics important for ensuring these requirements. Further, from a view point of an increase in sensitivity of a head, the soft magnetic characteristics play an important role. In order to increase the sensitivity, the direction of magnetization of the free layer must be rotated with a limited amount of magnetic flux of a recording medium, so that a thicker free layer is not desirable.
Also, while the above-mentioned specifications etc. disclose a general layer configuration for a free layer which exhibits a larger change in magnetoresistance, they do not clarify a preferred layer configuration in consideration of the characteristic of change in magnetoresistance, and soft magnetic characteristics such as magnetostriction.